Abstract Hematopoiesis relies on the proper function of hematopoietic stem and progenitor cells (HSPCs), in their capacity for both self-renewal and differentiation. Genetic lesions in these cell populations can give rise to myeloid malignancies including myeloproliferative neoplasms (MPN) and acute myeloid leukemia (AML). Myeloproliferative neoplasms, which result in the expansion of mature myeloid compartments, commonly harbor aberrations or mutations involving the JAK/STAT signaling pathway. Mutations in JAK2, CALR, and MPL are found in the majority of MPN cases, however cooperating co-driver mutations and disease modifiers are relatively poorly understood. Current treatment options for Philadelphia-chromosome-negative (Ph-) MPNs provide symptomatic relief and do not significantly alter overall survival. Moreover, leukemic transformation to AML is a common occurrence in Ph- MPN patients, and is thought to be due to acquisition of additional mutations which ultimately lead to a dramatic reduction in patient survival. The process of leukemic transformation of MPN to AML still remains unclear, and gaining insight into the molecular mechanisms of this phenomenon may provide openings for new interventions for preventing disease progression. We have discovered that loss of the Nol3 gene in mice leads to an MPN-like disease closely resembling primary myelofibrosis (PMF). Paradoxical to its canonical functions in repressing apoptosis, deletion of Nol3 results in an increased expansion and cycling of HSPCs in the bone marrow and spleen, suggesting a novel role for Nol3 within the hematopoietic system. Our analyses show that Nol3 is frequently downregulated or deleted in patients with PMF and other myeloid malignancies, and our data provides a link between Nol3 expression and JAK/STAT activation in MPN and AML cells, implicating Nol3 in the JAK/STAT signaling pathway. Our proposal seeks to (1) characterize the molecular role of Nol3 in hematopoiesis by defining molecular functions and subcellular localization, as well as identifying functional binding partners, (2) relate Nol3 loss to human disease by dissecting the role of Nol3 within the JAK/STAT pathway and identifying critical functional domains, and (3) gain insight into cooperativity between Nol3 loss and mutational drivers in MPN and transformation to AML. Our study will not only provide a novel and robust model for studying myeloid malignancies, but will also help define for the first time a role for Nol3 as a myeloid tumor suppressor and key component of the JAK/STAT signaling pathway. More importantly, our studies will also link Nol3 to human myeloid malignancy, which will provide a deeper understanding of the molecular pathogenesis of disease.